On 12/21/2016 10:06 AM, Tom DalPozzo wrote:
Is there an index on this table?

Have you tried a REINDEX on it?

yes there is an index on id field. I tried REINDEX. Nothing changes but
I notice now (but perhaps it was like that even before reindexing) that
every time I issue that UPDATE query, the number of the block it can't
read increases by one. Now, after some attempts: ERROR: could not read
block 12289 in file "base/16384/29153": read only 0 of 8192 bytes.

Unfortunately I don't remember every step as I was focused on completely
other things... Anyway, in synthesis:
1 pg_basebackup on primary and added, to the just created backup pg_xlog
dir, the needed WAL files according to the .label file (I'm trying
without archiving) .

A new thing:
I noticed that, always restarting from the corrupted cluster (without
reindex I mean), if I update the row id=409 with few data (3 bytes),
then it works and after that, even updating with that long data works.

To me that looks like an issue with the associated TOAST table. I do not
have a suggestion at this time. Maybe this rings a bell with someone else.